1. Field of the Invention
The present invention relates to an implantable medical device, more particularly an implantable cardiac device, such as a cardiac pacemaker or defibrillator.
2. Background Art
Implantable devices of this type generally incorporate a central control unit for controlling the internal and external functions of the device. In concrete terms for a cardiac pacemaker this means that a microprocessor, for example, assumes the internal pacemaker control processes, the processing of the EKG data obtained from measuring electrodes in the heart and the control of driver units for the delivery of electrical stimulation pulses to the heart.
To perform these functions, the central control unit requires, among others, a memory unit comprising program memory, main memory and/or data memory. The design of and need for the different types of memory depends on the functions of the implant.
Lastly, based on the nature of the location where these types of implantable devices are used, an off-the-line power supply must be provided for them, e.g., in the form of a battery.
A central problem with conventional implantable devices as they have been used until now, is the fact that semiconductor memory chips are used as the memory units. It is true that these semiconductor memory chips in themselves provide sufficient storage capacity to support also complex functions of the implantable devices. However, memory chips of this type also have a considerable current consumption, which, in practice, for devices that need to be implanted for the long term, limits the size of the memory chips that can be used. This, in turn, limits the functionality of the device, which, of course, counteracts the development trends towards an increasingly complex spectrum of functions for this type of implant. Furthermore, while semiconductor memories on the basis of submicron technologies, which are increasingly gaining importance, do require less and less space, their statistical power consumption, on the other hand, is growing superproportionally.
It is true that storage media are basically known from the prior art that operate based on magnetic effects virtually without power consumption and that can, therefore, be considered non-volatile storage. However, these have proven to be of little use for the relevant implantable devices because of their limited storage capacity, which is caused by other reasons.
It is an object of the invention to provide for an implantable medial device comprising a memory with low power consumption and high storage capacity.
To achieve this object, the present invention now proposes to use non-volatile read-write memories for the memory unit that are based on an optical storage medium and maintain the stored information energy-free.
This type of memory has significant advantages, particularly in view of its application in implantable devices that need to be operated off-the-line.
Energy is consumed merely for reading or changing the memory contents, but not for maintaining the information. This drastically reduces the energy consumption of an implant of this type so that, conversely, at a given pre-defined energy capacity, the storage capacity can be increased. Furthermore, optical memories may be implemented in very different configurations, which may be specifically tailored to their application in implantable medical devices. This variability does not exist to this extent in the silicon memory technology.
According to a first preferred variation for the optical storage medium, the latter may be a polymer with optically excitable molecule structures. The function of a memory cell is thus based, for example, on the orientation of a molecule in the polymer material, which is changeable by emitting light onto the memory medium. The given orientation may be reflected in optical properties, such as, e.g., the polarity or the absorption coefficient of the material. It is via these physical properties that the information can be scanned from outside. The writing and reading of information may take place especially by means of emitting different optical pulses onto the memory medium. Pulses below the limit value, for example, may be used to scan the absorption coefficient of an optical memory cell and thus their information contents. If an optical pulse above the defined limit value is emitted onto the memory medium, the molecule structure is excitable and can accept a new information content, e.g., in the form of a changed absorption coefficient, through a change in its orientation.
The control circuit of a polymeric read-write memory of this type may be advantageously implemented with polymeric transistors in such a way that the interface between the silicon based functional blocks of the implant and the optical memory unit may be reduced to a few contacts.
As a second preferred alternative for an optical storage medium, the invention comprises a semiconductor material with optically excitable energy sinks in a conductance band. According to this concept, electrons can first be raised into the conductance band by means of optical excitation processes, where they are maintained energy-free by the energy sinks. A memory cell can thus be implemented in this manner as well, in such a way that the scanning of the information and raising of the electrons into the conductance band and, vice versa, its inverse transformation to the initial state, again takes place via the emittance of light of a certain wavelength onto the memory medium.
Further characteristics, details and advantages of the invention will become apparent from the following description, in which an embodiment will be explained in greater detail based on the appended drawing.